The present invention relates to the hydrolysis of chlorosilanes including organochlorosilanes in a stoichiometric amount of water to produce a polysiloxane hydrolyzate and anhydrous hydrogen chloride. More particularly, the present invention relates to the hydrolysis. of dimethyldichlorosilane resulting in the production of dimethylpolysiloxane hydrolyzate, anhydrous hydrogen chloride and a saturated aqueous hydrogen chloride solution which can be recycled.
Prior to the present invention, dimethyldichlorosilane was hydrolyzed in the presence of an excess amount of water to produce a dimethylpolysiloxane hydrolyzate consisting essentially of a major amount of dimethylcyclopolysiloxane and a substantially linear silanol terminated dimethylpolysiloxane. An aqueous solution of hydrogen chloride was also formed which contained a significant amount of salvageable hydrogen chloride which could be used to convert methanol to methyl chloride used in the direct method for making dimethyldichlorosilane.
One procedure for recovering the hydrogen chloride from the aqueous hydrochloric acid was to distill the mixture to produce a constant boiling HCl-H.sub.2 O azeotrope along with anhydrous hydrogen chloride. A significant amount of energy is required in the distillation to salvage as much of the available hydrogen chloride from the dimethyldichlorosilane hydrolysis. In An Introduction to the Chemistry of the Silicones, 2nd Edition, (1951), John Wiley & Sons, Inc., New York, E.G. Rochow indicate that, if insufficient water is supplied without a mutual solvent, part of the dimethyldichlorosilane is hydrolyzed completely to dimethylpolysiloxane and part does not react at all, or end up as linear polysiloxane with terminal halogen atoms. In addition, if hydrolysis is conducted in the presence of excess water, significant amounts of heat are generated exothermically which can result in processing problems in particular situations.
The present invention is based on the discovery that if substantially stoichiometric amounts of water are utilized to hydrolyze dimethyldichlorosilane, that anhydrous hydrogen chloride is generated directly along with a saturated solution of hydrogen chloride and water. It has been further found that additional benefits are achieved if the aqueous saturated hydrogen chloride solution is recycled to the hydrolysis reactor. In addition, it has been found that the resulting dimethylpolysiloxane is substantially the same dimethylpolysiloxane hydrolyzate produced by the procedure of the prior art utilizing excess water. Further, the method of the present invention provides anhydrous hydrogen chloride which separates from the hydrolysis mixture as it is formed during the reaction. In essence, the reactor is used as both a theoretical distillation plate and chlorosilane hydrolysis reactor.